1. Field of the Invention
The present invention relates to a communication apparatus that performs data communication, such as communication of image data and text data, by using a modem having a mode in which a data receiving operation is automatically shifted to a control signal communication upon detection of a training signal pattern during the data receiving operation. The invention also relates to a communication method used in the above communication apparatus.
2. Description of the Related Art
Along with the increased transmission rate of modems, it is possible that 28.8-bps modems in compliance with ITU-T Recommendation V.34 can be used even in communication apparatuses, such as facsimile machines. In performing facsimile communication by using a modem having such a high transmission rate, V.34 communication image data is formed of a continuous superframe (280 ms). Accordingly, if a line signal is momentarily disconnected while communication of image signals is being performed, the synchronization of superframe communications is lost, and image data cannot be received until the synchronization of superframe communications is recovered. What is worse, processing for providing resynchronization after the line signal is interrupted is technically complicated, and resynchronization is substantially impossible to provide if the line signal is disconnected for 30 ms or longer, thereby failing to correctly receive the subsequent image data.
Some V.34 modems have the following mode: upon detection of a training signal pattern while an image signal is being received, the operation is automatically shifted from the receiving of the image signal to the training signal. If the above-described mode is disabled, even if a signal pattern similar to a training signal is detected while an image signal is being received, the operation is not shifted to the receiving of the training signal but continues to receive the image signal. This makes it possible to hold the image signal in the same channel as the one used before the receiving of the image signal was interrupted. Thus, if the signal line is disconnected only for a very short time, the image signal continues to be received after synchronization is re-established. However, when the foregoing mode is disabled, communication is shifted to a control channel (control signals) only when a return-to-control-procedure (RCP) signal (a signal for shifting communications to a control channel) added to the end (the end of a block) of an image signal is detected. Thus, if a signal is disconnected for 30 ms or longer, the modem is unable to detect the RCP signal and to perform demodulation, thereby failing to shift the communications to the control channel. Thus, the subsequent process cannot be performed, and the communications are ended erroneously.
Conversely, if the foregoing mode is enabled, communication is shifted from the receiving operation of an image signal to a control channel even if an RCP signal is not correctly received and detected, thereby making it possible to perform the subsequent process. On the other hand, if a pattern similar to a training signal is contained in the received image signal, the modem erroneously detects it as a training signal and is unable to perform demodulation by initializing an equalizer of the modem. This makes it impossible for the modem to receive the subsequent image signals. Further, any momentary disconnection of a signal immediately causes the communication to shift from the receiving operation of the image signal to the control signal. Accordingly, the time limit may be reached while communication is still being made.
In this manner, data communication performed by using modems having a high transmission rate, such as V.34 modems, is more vulnerable to momentary disconnection of a line signal than those by using conventional V.17 modems having a 14.4 kbps or lower rate, thereby causing more communications errors.